IMMERSE plank in water, placing weight on top of plank to keep it submerged. Soak 4 hours.
HEAT barbecue to medium heat. Place plank on grate of barbecue; close lid. Heat plank 5 min. or until it starts to crackle and smoke. Brush with oil.
MIX dressing and syrup; pour over salmon in shallow dish. Turn salmon over to evenly coat both sides. Remove salmon from dressing mixture; discard dressing mixture. Place salmon on plank; top with onions. Cover barbecue with lid.
GRILL 20 min. or until salmon flakes easily with fork. Cut into 8 pieces to serve.